As far as I know, none of Akaishi’s works are officially considered horror, but many qualify as mystery or suspense (and some are easily as bloody as recognized horror titles). Ten yori mo Hoshi yori mo ("Than the Heavens, than the Stars") is probably the most popular of these "mysterious romances." It is a good series, especially if you’re familiar with the relevent history and legends. Obviously I like Asterisk even more, although my absolute favorite of the mystery genre is actually the short story Soshite Tenshi wa Hohoenda ("And the Angel Smiled").
All of Akaishi’s works have the characteristics most readers associate with girls’ comics: a softer, less realistic art style and a strong romantic bent. But they also have a delicacy surprising to those readers more familiar with the histrionic soap operas which are all too common in any media intended for girls.
Her official home page is Penguin Room, with both Japanese and English versions on-line.